Fabrication of polydimethylsiloxane optical material

ABSTRACT

A lithography-free, mold-free method of fabricating high quality optical material by curing polydimethylsiloxane (PDMS) droplets in or on pre-heated substrates allows lenses with different focal lengths to be made by varying the volume and surface temperature, as well as the substrate.

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 15/977,019, entitled “Fabrication ofPolydimethylsiloxane Material,” filed May 11, 2018, which is acontinuation-in-part of and claims priority to U.S. patent applicationSer. No. 14/813,728, entitled “Fabrication of Lenses by DropletFormation on a Pre-Heated Surface,” filed on Jul. 30, 2015, which claimspriority to U.S. Provisional Patent Application No. 62/031,516, entitled“Fabrication of Lenses by Droplet Formation on a Pre-Heated Surface,”filed on Jul. 31, 2014, the entire contents of which are herebyincorporated by reference.

The present invention used in part funds from the National ScienceFoundation (NSF) CAREER Award No. CBET-1151154, the National Aeronauticsand Space Administration (NASA) Early Career Faculty Grant No.NNX12AQ44G) and the Gulf of Mexico Research Initiative Grant No.GoMRI-030. The United States Government has certain rights in theinvention.

BACKGROUND

This disclosure pertains to method for fabricating high quality opticalmaterial, such as lenses and fibers, by curing liquidpolydimethylsiloxane (PDMS) in or on heated substrates.

Lenses are traditionally constructed with rigid materials such as glassor plastics by mechanical polishing or injection molding. High opticalquality lens surface requires well-controlled fabrication parameterswhich increases complexity and reduces yield. Current demands forcomplementary metal-oxide semiconductor (CMOS) image sensors haveresulted in the increase in fabricating small lenses ranging from 1 mmto 1 cm in diameter. In addition, emerging applications of flexibleoptoelectronics demand mechanically flexible lens materials. Fluidiclenses in particular is a simple method of creating small lenses of highquality without the requirement of molds or complex parameter control.However, an encapsulated fluidic lens requires a system to providemechanical stability, and prevent evaporation. In contrast, lensformation due to surface energy minimization during polymer curing hasprovided an alternative method for making high quality, low-cost“fluidic” lenses that are independent components, flexible and robust.

Polymers have been generally utilized as a lens material by threecategories of fabrication techniques: 1) lithographic methods, 2)surface-tension-driven methods, and 3) imprinting or embossing methods.These approaches demonstrate the feasibility of creating lenses withgood optical characteristics and reproducibility; however, thesetechniques involve either time-consuming fabrication procedurestypically measured in hours, or have high costs due to lithographic ormolding equipment required, and generally limit the size of the lens tothe micrometer scale. A recently introduced alternative method ofcreating a lens by droplet formation requires iterative drop-bake cyclesto achieve a desired focal length. What is needed is a method for theproduction of high quality, inexpensive lenses with optimal focal lengththat requires minimal steps and is low cost.

SUMMARY

The present disclosure relates generally to a method for fabricatinglithography-free, mold-free, inexpensive, and high qualitypolydimethylsiloxane (PDMS) optical lenses and fibers by curing liquidPDMS on a pre-heated smooth surface, in or on a pre-heated liquid, or ina pre-heated gaseous plane. Current methods for fabricating mold-freepolydimethylsiloxane (PDMS) have relied on iterative gravity-assistedprocesses. These techniques can produce high quality lenses, but requireiterative steps to provide optimal focal length. The present methodproduces inexpensive, high quality optical material in a variety ofshapes and cross-sections. The focal length of each lens can be variedby changing the volume of PDMS, the temperature of the curing substrate,the motion of the substrate relative to the dispensing of the PDMS, andthe like. With these methods, in some embodiments, a focal length asshort as 0.5 mm and as long as 10 cm can be achieved. Furthermore, byattaching a preferred embodiment of a lens on a smartphone camera, animaging resolution of 10 μm, and as small as 2 μm, is possible. Lensesof different shapes and formations can be created, as well as opticalfibers.

The process for fabricating optical material requires simply ejecting avolume of polydimethylsiloxane (PDMS) in liquid form into or onto asubstrate having a pre-selected temperature and allowing the PDMS tocure to solid form. The material as cured may have a diameter and afocal length, or a cross-section of the cured PDMS may have acontrollable lens shape. The volume of PDMS, the substrate, and thetemperature are selected to optimize the features of the opticalmaterial, and the process in certain embodiments requires no furthersteps or iterations to produce the optical lens.

Polydimethylsiloxane (PDMS) is optically transparent (T>95%) in thevisible spectrum with high refractive index (n=1.47˜1.55), and displaysminimal yellowing over time. Preferred embodiments of this disclosurerelate to a method to manufacture lenses by curing PDMS in or on apre-heated substrate. When PDMS is dropped onto a surface, interfacialsurface energies allow the droplet to hold itself up into a dropletshape which naturally acts as a lens. By controlling the nature of thesubstrate, the volume of the droplet, and the temperature of thesubstrate, the speed of curing can be controlled, which allows the PDMSto solidify while still retaining a curvature, thus form lenses ofdifferent focal lengths. This method produces lenses having the abilityto transform any mobile camera device into a microscope, or to add anarea with additional magnification to a pair of eyeglasses. The strong,but non-permanent adhesion between PDMS and glass allows the lens to beeasily detached or replaced after use without supporting structures. Animaging resolution of 10 μm, and an optical magnification of ×12 hasbeen demonstrated.

The quality of optical material depends on its geometry and surfacesmoothness. Fluidic lenses made by a droplet of solution can form veryhigh quality lenses, as the surface tension distributes evenly on thedroplet. Similarly, when uncured liquid PDMS is dropped or dispensedonto a surface, it assumes a convex shape and can be used as a fluidiclens. However, when the surface or substrate is not heated, therheological properties of PDMS cause the droplet to spread out onto thesurface until equilibrium is attained between the interfacial surfaceenergies and gravity. The subsequent lens may be very thin and wide,with a very large radius of curvature, and little lensing effect.

Because PDMS is a thermally curable elastomer, the flow of the PDMS canbe limited by decreasing the time allowed for the material to cure. Alonger curing time allows more time for the PDMS to flow into a thinpancake-like structure, while a shorter curing time prevents excessflowing, as shown in FIG. 1. Two variables are important, thetemperature of the substrate on or in which a volume of PDMS is ejected,and the volume of PDMS ejected or deposited. The substrate itself canalso be varied. By controlling these variables, the geometry,cross-section and focal length of the lens or fiber can be fine-tuned.In additional embodiments, dyes and nanoparticles can be incorporatedinto the PDMS prior to curing. In additional embodiments, the mechanismfor ejecting the PDMS is moved relative the surface during deposition tovary the shape of the material. In further embodiments, a heated liquidsurface is used as the substrate. In further embodiments, while the PDMSis being ejected, it passes through a heated gaseous plane to form afiber shape. In additional further embodiments, water is injected intoliquid PDMS deposited on a heated substrate to produce a concave lens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows (a) an image of droplets of polydimethylsiloxane (PDMS)deposited on a coverslip with a syringe and (b) a representation of howchanging the temperature of the pre-heated surface modifies the geometryof a cured PDMS droplet.

FIG. 2 shows an image of optical material formed to have an extendedcylindrical lens shape in accordance with preferred embodimentsdescribed herein.

FIG. 3 shows an image of optical material having an irregular orfree-form extended cylindrical lens shape in accordance with preferredembodiments described herein.

FIG. 4 shows an image of an optical lens having a generally sphericalball-shape or rounded shape in accordance with preferred embodimentsdescribed herein.

FIG. 5 shows a diagram of an exemplary arrangement in which PDMS isinjected into a fluid substrate onto a heated surface.

FIG. 6 shows an image of convex lenses of increased diameter inaccordance with preferred embodiments described herein.

FIG. 7 shows images of 50 μL PDMS droplets dropped on a surface attemperatures of (a) 60° C., (b) 80° C., (c) 100° C., (d) 120° C., (e)140° C., (f) 160° C., (g) 180° C., (h), 200° C., demonstrating theeffect of speed-curing of PDMS to form lenses.

FIG. 8 shows (a) the setup for imaging an LCD with a smartphone camerawith a PDMS lens positioned in between to demonstrate changes in lensmagnification for PDMS lenses dropped on a surface at varyingtemperatures and the resulting magnification with (b) no lens and attemperatures of (c) 60° C., (d) 80° C., (e) 100° C., (f) 120° C., (g)140° C., (h) 160° C., (i) 180° C., and (j) 200° C.

FIG. 9 shows images of uncured PDMS dropped onto a surface of 200° C.with a volume of (a) 50, (b) 75, (c) 100, (d) 125, (e) 150, (f) 175, and(g) 200 μL.

FIG. 10 shows change in focal length for PDMS lenses in response tovariations in

-   -   (a) surface temperature, and (b) droplet volume.

FIG. 11 shows (a) an image of a PDMS lens directly and non-permanentlybonded onto the lens element of a smartphone camera, (b) an image of afingerprint with inset showing magnified image with and without lens,(c) an image of a spider with inset showing magnified portion oftrichobotria, and (d) an image of a PenTile Matrix OLED screen withinsets showing actual structure design, and the same structure takenwithout the lens.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present disclosure relates to a mold-free method of manufacturingoptical material by curing polydimethylsiloxane (PDMS) in or on a heatedsubstrate. This method allows optical material with different sizes,shapes, and focal lengths to be made by varying the PDMS volume, thesubstrate composition and temperature, the distance or positioningbetween the PDMS and the surface, and other factors. In an initial step,a volume of polydimethylsiloxane (PDMS) in liquid droplet form isejected from a deposition mechanism such as a nozzle onto a substratehaving a pre-selected temperature or into a fluid (gaseous or liquid)substrate having a pre-selected temperature. In a subsequent step, thePDMS is allowed to cure to solid form. The cured lens material mayitself form a lens having a diameter and a focal length, wherein thepre-selected volume and the pre-selected temperature are selected tooptimize the diameter and the focal length of the lens. In alternateembodiments, the cured lens material may have a cross-section that canbe utilized as a lens, or it may form an optical fiber.

In certain embodiments, the preferred PDMS volume is between 0.1 μL and10 mL. Preferred surface temperature is between 60° C. and 300° C.,preferably about 200° C. The diameter of the resulting lens ispreferably 2 cm or less in some embodiments and can be greater than 2 cmin other embodiments. The focal length of the resulting lens is between0.5 mm and 10 cm, and preferably about 6 mm. An imaging resolution of 10μm, and an optical magnification of ×12 has been demonstrated. Themethod can also be modified for parallel fabrication that may allowhigher throughput. The material cost of the PDMS was calculated to be<$0.01 USD for a 50 μL lens, and can be conveniently attached to amobile camera or any vision-enhancing lenses via the strong butnon-permanent adhesion between PDMS and glass or plastic. The cured PDMSlenses can effectively act in certain embodiments as a supplemental lensthat improves the magnification and performance of other lenses.Preferred embodiments include lenses for attachment to the mobile cameramodules of any smartphones, tablets, smartwatches, or laptops, as wellas lenses for attachment to any wearable products such as eyeglasses orenhanced “smart” glasses. For eyeglasses and wearable products, lensesthat are larger work better. When the lenses are attached onto eyespectacles, they can provide the viewer a magnified view of a portion onthe spectacle when the object is held at the focal length of the lens,0.5 mm to 10 cm. No external accessories or attachments are required toobtain the increased magnification for any of the preferredapplications, other than the lens itself.

Certain preferred embodiments relate to a method for manufacturingcylindrical lenses using the same general methods described herein. Inthis embodiment, the volume of dispensed PDMS is continuous and notlimited. In addition, one axis of the heated surface is moved relativeto the ejection point or the nozzle, for example, the x-axis. Theheat-assisted in-situ curing causes the liquid PDMS deposited on surfaceto cure similarly as described above. However, as the PDMS dispensing iscontinuous, a long cylindrical shape is formed in the cured lensmaterial. The cross-section of the cured lens material has acontrollable lens shape, as shown in FIG. 2. In this embodiment, theheated smooth surface may be moved in a straight line along a singleaxis relative to the ejection point of the PDMS, and the lens materialis formed to have an extended cylindrical lens shape.

Certain additional preferred embodiments relate to a method formanufacturing free-form lenses that do not have a regular convex orcylindrical shape. In this embodiment, similar to the method ofmanufacturing cylindrical lenses, the volume of dispensed PDMS iscontinuous and not limited. In this embodiment, the heated surface ismoved in any direction. The movement of surface motion relative to theejection point is not limited to a single axis and direction. The motioncan occur in both axes and in different directions to create a printedpattern whereby the cross-section of the material consists of acontrollable lens shape, as shown in FIG. 3. In this embodiment, theheated smooth surface is moved in an irregular fashion along more thanone axis relative to the ejection point, and the lens material is formedto have an irregular extended cylindrical lens shape. For example, ifthe surface is moved in an x-y direction, a perpendicular cross-sectionwill have the same perpendicular cross-section as a droplet lens.

In further preferred embodiments, the liquid PDMS is ejected into oronto a heated fluid substrate, which may be a heated liquid substrate ora heated gaseous substrate.

Additional preferred embodiments relate to a method for manufacturingball shaped lenses. In this embodiment, PDMS is deposited into a heatedliquid substrate such as water so that it is submerged. In thisembodiment, any water-based (as opposed to oil-based) liquid can be usedas the heated liquid substrate. The ejection point or nozzle forejecting the PDMS can be submerged inside the liquid and near the bottomof the liquid container. In this embodiment, the liquid must be heatedwithout bubbling to enable the PDMS to cure without any bubble defectformation. It is preferable to keep the heated liquid substrate at atemperature below the liquid's nucleation temperature, which would leadto bubbling, so as not to agitate the mixture. Preferably thetemperature should be as high as possible, such as about 60° C. Anyvolume of heated fluid substrate can be used, so long as the volume islarge enough to produce the desired lens. The volume of heated fluidsubstrate may be up to about 10 mL. This method creates spherical oraspherical ball-shaped or rounded lenses similar to that shown in FIG.4.

Additional preferred embodiments relate to a method for manufacturingconvex lenses having increased diameters. In these methods, a PDMSdroplet is ejected from an ejection point above a heated liquidsubstrate and allowed to fall into the liquid substrate, or a PDMSdroplet is ejected from an ejection point within a heated liquidsubstrate and allowed to fall onto a heated surface at the bottom of theliquid. In these embodiments, the same types of liquid substrate can beused as discussed above, namely any suitable water-based liquid heatedto a temperature that is as high as possible without reaching theliquid's nucleation temperature. In certain preferred embodiments, theliquid substrate may be in a vessel above a heated surface and acontrolled volume of PDMS ejected within the liquid may pass through theliquid substrate over a constant drop height onto the heated surface, asshown in FIG. 5. The resulting curvature of the lens is controlled bythe liquid substrate temperature, the volume of the PDMS, thetemperature of the surface underneath the liquid substrate, and theadditional force of the surface tension of the liquid substrate. In thisembodiment, the volume of liquid substrate can be as large as necessaryto obtain a significant curvature in the lens, or up to about 2 mL. Thismethod creates convex lenses of increased diameter similar to that shownin FIG. 6. In these embodiments, the lenses may have a diameter of 2 cmor greater.

Additional preferred embodiments relate to a method for manufacturing anoptical fiber by PDMS “trail pulling” and instantaneous heating in agaseous plane. In this embodiment, the PDMS is continuously ejected, andinstead of being deposited on a heated substrate, the material insteadpasses through a heated gaseous plane. Any non-flammable gas can beused, such as air or purified nitrogen. There is no preference on thegas, as it is only providing heating from a heat source that creates theheated gaseous plane. The temperature must be high enough to enableinstantaneous PDMS curing (less than about 100 ms curing time). Thus, inpreferred embodiment, the gaseous plane should be at a temperature ofabout 200° C. or higher. The volume of PDMS ejected is continuous.However, the PDMS trail should be as thin as desired to be used in anoptical fiber configuration. Accordingly, in preferred embodiments thediameter of the PDMS trail should be no larger than 1 mm. The heatedgaseous plane cures the PDMS in a fiber shape and the fiber can becollected beneath the plane directly or by rolling into a reel.

Additional preferred embodiments relate to a method for manufacturing aconcave lens in accordance with the general methods described herein. Inthis method, a portion of PDMS is ejected onto a heated substrate.Subsequently, a liquid such as water is injected into at least oneposition in the PDMS to adjust the final shape taken by the lensmaterial after curing. Liquid injected that is in contact with thesubstrate surface within the PDMS will cause the PDMS to take on aconcave shape as it cures around the injected liquid. The curvature andoptical power of the lens that is formed will depend on the differentsurface tensions between the PDMS and the liquid and gravity, and willalso depend on the volume of the PDMS relative to the volume of injectedliquid.

In certain additional embodiments, prior to depositing and curing thePDMS droplet, selected particles, such as dyes or nanoparticles, can beincorporated into the liquid PDMS. The particles could include foodcoloring, such as Blue #1, Yellow #5, Red #40, and various mixture oftwo or more of these. All colors of food coloring can be incorporatedinto PDMS because the other colors are simply a mixture of these colors.These food colorings can either be water- or oil-based. In certainembodiments, the PDMS can be embedded with colored apolar dyes forwavelength filtration, such as Nile Red, Sudan Red, and the like. ThePDMS can also be embedded with colored polar dyes for wavelengthfiltration. In addition, titanium dioxide could be added to the PDMS.This will create a white opaque lens. Other exemplary particles are:Rhodamine 6G, which creates a fluorescent orange/green colored lens;Crystal Violet, which creates a purple colored lens; Methylene Blue,which creates a blue colored lens; Acridine Orange, which creates anorange colored lens; Gold nanoparticles, which create various colorsdepending on the diameter of the gold nanoparticles; and Iron powder,which creates a grey opaque lens. Additional examples include Cy3, Cy5,Cy7, Cy9, Alexa fluor dyes, silver nanoparticles, CdSe and ZnSe quantumdots, photochromic materials, and infrared transparent materials such assilicon particles, germanium particles, and chalcogenide compounds. Allpowdered or liquid dyes, as well as colored or opaque nanoparticles,quantum dots or other colored materials, could be added to the liquidPDMS prior to curing. Magnetic nanoparticles can also be embedded in thePDMS.

Generally speaking, in order to incorporate powdered particles into theliquid PDMS, the first step is to dissolve the particles in water, orethanol, or another solvent that does not interfere with the particles.If the particles are already liquid-based, such as either water oroil-based, this step is not necessary. The particle-liquid mixture isthen mixed into uncured liquid PDMS and degassing of the mixture isperformed. Then, the same procedure is followed as with un-modifiedliquid PDMS. The formation of colored lenses facilitates the use ofthese lenses in not only magnification, but also measuring colorimetricchanges in samples of objects. The lenses allow for wavelength selectiveelectromagnetic wave filtering. For example, differently colored lensescan be used to observe the same microscopic samples and obtain relativeenhancement or inhibition of different wavelengths.

Example 1. Temperature Characterization

PDMS solution (SYLGARD® 184, Dow Corning) was prepared by mixingmanufacture recommended proportions of PDMS base and curing agent by aweight ratio of 10:1. After mixing and vacuum bubble removal, a syringewas used to deposit PDMS on the surface.

To prepare a pre-heated surface, a coverslip was cleaned and placed ontop of a hotplate set to 60° C., and an infrared camera was used toverify that the coverslip glass reaches equilibrium temperature. 50 μLof PDMS was dropped on the coverslip from a 2 cm height with thesyringe. PDMS cures at room temperature, but exhibits accelerated curingwith increasing temperatures. The procedure was repeated with thehotplate temperature set to 80° C., 100° C., 120° C., 140° C., 160° C.,180° C., and 200° C. respectively, with the resulting figures andproperties of the droplet shown in FIG. 7. In each case, an infraredcamera was employed to verify the coverslip has reached equilibriumtemperature.

At 60° C., the PDMS droplet requires >5 minutes to cure, the longercuring time allows the PDMS to spread across the surface to form a verythin, flat lens with negligible magnification. With increasingtemperature, the curing time decreases, thus limiting the flow of thedroplet and creates a smaller diameter lens. The focal length decreasesdue to the increase in surface curvature, which increases themagnification of the lens. At a higher temperature of 200° C., the PDMSdroplet requires <5 seconds to cure, the droplet cures shortly oncontact and exhibits a very high radius of curvature and enhancedmagnification. It is significant to note that at this high temperature,no burning or charring was observed.

The optical power of a convex lens is the degree to which it convergeslight, and can be expressed by the equation P=1/f, where P is the powerof the lens, and f is the focal length. FIG. 8 demonstrates therelationship between the optical power of the lens and the curingtemperature, the lens is placed on a coverslip glass and positionedbetween a computer LCD monitor displaying a white screen, and asmartphone camera capturing the image. It is seen that with increasingtemperature of curing, the optical power increases as the focal lengthdecreases.

Example 2. Volume Characterization

To investigate the effect on the focal length and magnification on thedrop volume, the hotplate was set to a constant temperature of 200° C.and different volumes of PDMS were dropped onto the surface, with allother conditions remaining identical to those described in Example 1above. Volumes of 50, 75, 100, 125, 150, 175, and 200 μL were usedrespectively, with the resulting figures and properties shown in FIG. 9.

As the droplet volume increases, the lens diameter increases, howeverthe contact angle only varies within ±4.4%. The center region of thelens is found to exhibit decreasing curvature, and the focal length isfound to increase slightly between 50 μL and 100 μL, from 6 mm to 8 mmrespectively, and increases significantly between 100 μL and 200 μL,from 12 mm to 60 mm respectively. This can be shown that as the size ofthe droplet increases, the radius of curvature near the center of thelens no longer increases, and with an increasing volume of the PDMSdroplet, the center portion will eventually become flat. This limitationmakes it difficult using this method to create practical lenses largerthan 2 cm in diameter.

The properties of the lens are determined by its geometric parameters.Since the curing temperature of PDMS determines the speed of curing, andthus the contact angle and diameter of the lens droplet, the focallength of the lens can be accurately tuned in a single-step. As withfluidic lenses, mechanical defects on the curved surface are eliminatedwith this mold-free process, as the droplet experiences uniform surfaceforces from all sides during curing. FIG. 10 shows the change in focallength with response to (a) surface temperature, and (b) droplet volume.

Example 3. Numerical Aperture

From the preceding examples, it is shown that the lens created with a 50μL volume droplet size, cured at 200° C. is most suitable forshort-ranged imaging due to its short focal length. This exampleattempts to characterize the numerical aperture of this particular lens.The focal length of the lens is found to be 6 mm by focusing a whitelight onto a white plastic board. A 1-watt white LED flashlight sourcewas positioned 1 m away from the lens, and a white plastic board on theother side of the lens was moved slowly towards the lens until a focusedimage was observed. The maximum usable curvature of the 6.0 mm diameterlens was found by superimposing a curve-fitted ellipse and taking thelength where the lens outline is within ±1% of the ellipse, which yieldsa maximum usable curvature of roughly 3.8 mm. The numerical aperture ofa lens is expressed as NA=n sin θ, where n is the index of refraction ofthe medium in which the lens is working (n_(air)=1), and θ is thehalf-angle of the maximum cone of light that can enter or exit the lens.For a focal length of 6 mm, and an effective radius of 1.9 mm, thehalf-angle is found to be 17.571°, and the numerical aperture is foundto be 0.422.

Example 4. Use of Lenses with Smartphones

Add-on accessories to turn a smartphone into a magnifying device arewidely available; however, most of these attachments significantlyincrease the bulk of the device. To demonstrate a practical application,the PDMS lens (50 μL cured at 200° C.) was attached to a Nokia Lumia 520budget smartphone camera (Microsoft, Redmond, Wash., USA) with a5-megapixel camera as shown in FIG. 11(a). The adhesive property of PDMSon glass and plastic surfaces allows the PDMS lens to bindnon-permanently onto the camera lens without supporting structures, andis not prone to falling off.

Images taken from the smartphone with no further image processing areshown in FIG. 11(b)-(d). FIG. 11(b) shows an image of a finger takenwith the camera without PDMS lens, and the inset taken with the PDMSlens attached clearly showing sub-millimeter fingerprint structuresincluding sweat pores. FIG. 11(c) shows an image of a spider taken withthe PDMS lens attached, and the inset shows a digitally magnifiedportion of the image. The thinnest thrichobotria structures on the legsof the spider, measure roughly 15 μm in width. FIG. 11(d) shows an imageof an organic light emitting diode (OLED) display with a PenTile matrixpixel geometry on a Nokia Lumia 800 smartphone displaying a whitescreen. The inset shows the actual geometry of the pixel matrix, thelarger pixel measuring 20 μm in width, the smaller pixel and gapsmeasure 10 μm in width. A minimum discernible resolution of roughly 10μm was achieved.

The magnification of the lens was found to be 12× by comparing theobserved size of an arbitrary structure in a clearly focused image takenwith the smartphone, and with a commercial microscope (Olympus IX-70,Olympus Corp) with 200× total magnification. The magnification of thesmartphone with PDMS lens can be further enhanced by combining softwarebased digital magnification.

Example 5. Use of Lenses with Eyeglasses

A 300 microliter lens was prepared at 200° C. as described above inExample 1, with a resulting magnification of 3×. The lens was attachedto a pair of eye spectacles via the natural adhesion of PDMS to thespectacle lens. The spectacle wearer was able to view items clearly infocus with a 3× magnification when looking through the spectacle withthe attached PDMS lens at any object when held at roughly 5 cm away fromthe spectacle.

Example 6. Focal Length

A 1 microliter lens was prepared at 200° C. as described above inExample 1, with a resulting focal length of 0.5 mm. A 1 mL (1000 μL)volume lens as also prepared at 200° C. as described above in Example 1.In this case, the resulting lens was completely flat in the middle withan infinite focal length. This lens did not distort or change thedirection of light and is actually useless as a lens. Although it ispossible to construct any range of focal length, up to an infinite focallength, the maximum usable range is approximately 10 cm. 10 cm is themaximum effective usable range for viewing microscopic samples.

Example 7. Color Effects

A red lens and a green lens were prepared according to Example 1 above,except that red and green dyes were incorporated into the liquid PDMSfor each lens prior to curing. The xylem/phloem of a plant histologicalcross section was observed with both lenses. With the red lens, thegreen objects (xylem) appeared enhanced, while the red objects (phloem)appeared suppressed. With the green lens, the opposite occurred. Thusthese lenses are useful as an image filter for enhancing/suppressingdesired wavelengths of light for easier identification of features andmeasuring of colorimetric changes.

What is claimed is:
 1. A method for fabricating an optical lens having aconcave shape, comprising: heating a substrate to a pre-selectedtemperature to produce a heated substrate; depositing a volume ofpolydimethylsiloxane (PDMS) in liquid form onto a surface of the heatedliquid substrate; injecting a volume of injection liquid into the volumeof polydimethylsiloxane (PDMS); and allowing the volume ofpolydimethylsiloxane (PDMS) to cure to solid form around the injectionliquid to create an optical lens having a concave shape, a diameter anda focal length, wherein the volume of polydimethylsiloxane (PDMS), thevolume of liquid, and the pre-selected temperature are selected tooptimize the diameter and the focal length of the optical lens.
 2. Themethod of claim 1, wherein the volume of polydimethylsiloxane (PDMS) isbetween 0.1 μL and 10 mL.
 3. The method of claim 1, wherein thepre-selected temperature is between 60° C. and 300° C.
 4. The method ofclaim 1, wherein the injection liquid is water.
 5. The method of claim1, wherein the volume of polydimethylsiloxane (PDMS) in liquid formfurther comprises particles incorporated into the PDMS.
 6. The method ofclaim 1, wherein the particles are dyes, nanoparticles, quantum dots,infrared transparent materials, or mixtures thereof.
 7. An optical lenshaving a concave shape prepared by the method of claim 1.